Aromatic Hydrocarbons: Explanation, Properties, Reactions, Uses, FAQs

Aromatic Hydrocarbons are defined as circularly structured organic compounds that contain sigma bonds along with delocalized pi electrons and they are also referred to as arenes or aryl hydrocarbons.

Aromatic Hydrocarbons Explanation

• Aromatic hydrocarbons are defined as “unsaturated hydrocarbons which have one or more planar six-carbon rings which are called benzene rings, to which hydrogen atoms are attached” and many aromatic hydrocarbons contain a benzene ring, also known as an aromatic ring. The benzene ring is always stabilized by resonance and the pi electrons are delocalized in the ring structure.

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  This Story also Contains

  1. Aromatic Hydrocarbons Explanation
  2. Properties of Aromatic Hydrocarbons
  3. Reactions of Aromatic Hydrocarbons
  4. Uses of Aromatic Hydrocarbons
  5. Polycyclic Aromatic Hydrocarbons

  

• It can be observed that all aromatic hydrocarbon compounds contain a benzene ring.

• The aromatic hydrocarbons which do not contain a benzene ring are commonly said as heteroarenes and all of these heteroarenes always obey Huckel’s rule

• Huckel’s rule states that the total number of pi electrons in a monocyclic ring is equal to 4n + 2 where n is any positive integer or zero.

• In these types of compounds, a minimum of one carbon is replaced by either nitrogen, oxygen, or sulphur and some examples of heteroarenes include furan which contains oxygen and pyridine which contains nitrogen.

Properties of Aromatic Hydrocarbons

The compound that was categorized first is an aromatic hydrocarbon called benzene and benzene is the most complex aryl hydrocarbon. Carbon atoms belonging to the benzene ring always have two carbon-carbon sigma bonds, one double bond and one carbon-hydrogen sigma bond with a neighbouring carbon where the pi-electron is always delocalized.

This delocalization process of pi electrons is represented by a circle in the benzene molecule inside the hexagon and all the carbon-carbon bond order in this molecule is considered to be 1.5 and this equivalency can be explained through the resonance structures of benzene.

The properties of aromatic hydrocarbons have been given below. They are-

• The properties of aromatic hydrocarbons exhibit aromaticity which means additional stability is granted by resonance.

• The ratio of hydrogen atoms to carbon atoms is always high in these molecule types.

• When it is burnt, the aromatic hydrocarbons exhibit a strong and sooty flame which results in yellow in colour.

• Aromatic hydrocarbon compounds only undergo electrophilic substitutions and nucleophilic aromatic substitution reactions.

• These compounds can be monocyclic or polycyclic.

Reactions of Aromatic Hydrocarbons

Many organic chemical reactions involve the use of aromatic hydrocarbons as the primary reactant and some such reactions are listed in this subsection along with a brief description of each of these reactions. They are-

1. Aromatic Substitution Reactions

Aromatic Substitution reactions involve the replacement of only one substituent on the ring of an aromatic hydrocarbon which is commonly called a hydrogen atom, by a different substituent group.

The common types of aromatic substitution reactions include below:

• Nucleophilic aromatic substitution reactions

• Electrophilic aromatic substitution reactions

• Radical nucleophilic aromatic substitution reaction.

Aromatic substitution reaction is the electrophilic substitution which can be observed in the nitration reaction of salicylic acid.

2. Coupling Reactions

In Coupling reactions, the coupling of two fragments which have a radical nature is achieved with the help of a metal catalyst and when aromatic hydrocarbons undergo coupling reactions, the following type of bonds can be formed. They are-

• Carbon-carbon bonds can be formed from the coupling reactions of arenes and products like vinyl arenes, alkyl arenes, etc. are formed.

• The formation of carbon-oxygen bonds can be formed in these reactions, by forming aryloxy compounds.

• Carbon-nitrogen bonds can form in coupling reactions which gives the products such as aniline.

3. Hydrogenation Reactions

The hydrogenation reactions involving arenes that are generally lead to the formation of saturated rings and an example of hydrogenation reactions is the reduction of 1-naphthol into a mixture which contains different isomers of decalin-ol.

Another example of hydrogenation reactions is the hydrogenation reaction of resorcinol with the help of spongy nickel which is also referred to as Raney nickel and aqueous NaOH. This reaction proceeds by the formation of an enolate, and the successive alkylation of this enolate with methyl iodide to yield 2-methyl-1,3-cyclohexanedione.

Uses of Aromatic Hydrocarbons

The use of aromatic hydrocarbons is in both biological and synthetic processes. Numerous uses of aromatic hydrocarbons are given below, they are-

• The green pigment found in plants, more commonly called chlorophyll, which consists of aromatic hydrocarbons and is very important in the process of food production in plants.

• The nucleic acids and amino acids in the human body also consist of the aromatic hydrocarbons.

• Methylbenzene which is an aromatic hydrocarbon is used as a solvent in model glues

• Naphthalene is the most important item in the production of mothballs

• Phenanthrene is used for the synthesis of drugs, dyes, and explosives, an aryl hydrocarbon.

• Trinitrotoluene or TNT is the most important aromatic hydrocarbon which is widely used for explosive purposes.

• The plastic industry and petrochemical industries also make use of aromatic hydrocarbons.

Polycyclic Aromatic Hydrocarbons

• Polycyclic Aromatic hydrocarbons are compounds which comprise aromatic rings in fused form and these are found in coal, tar, oil and some cooked foods like smoked fish, burnt toast, etc.

• One common example of these polycyclic hydrocarbons is naphthalene and these compounds are said to be pollutants.

• Examples of aromatic hydrocarbons are Methylbenzene, Naphthalene, Phenanthrene, Trinitrotoluene, and o-dihydroxybenzene.